Aircraft Physics Question

If a plane was flying along and it was carrying 5000kg of birds.

If all the birds flapped their wings at the same time and became airborne inside the cabin, would the plane now weigh 5000kg lighter?

 

Only if it took off from a conveyor belt.

Geffen

 

Do'h!!!

 

I'm interested in the answer too - I'm sure someone here knows the answer.

I had a variation of the OP question that goes like this;

If a fly was sitting on the inside wall of a sealed glass jar on top of very sensitive set of scales and suddenly became airborne within the jar, would the indicated weight be JAR WEIGHT - FLY WEIGHT? or.... would the downdraught created by the fly's wings to overcome gravity offset the weight of the fly?

 

the weight of the birds (cargo ) needs to be in contact with the hull of the plane for it to be measured / included as cargo / gross weight of the plane... absence of any contact with hull there are no forces ( from cargo ) acting on the the hull... as in the many videos showing weightlessness of the crew as it floats free of any contact with the hull when the plane is put into a dive... the reality is that the plane and cargo accelerate at different rates...if not secured to the hull, there is a slight delay between the two rates of acceleration, that the cargo becomes "weightless" with respect to the hull. To measure weight, an object must be in contact with something that offers resistance ( scales ) to measure the forces on it until force and resistance are in equilibrium. The plane would be lighter as soon as contact is lost

 

I thought maybe you were joking when you first asked. Of course, the joke could be on me. But in order for the birds to stay aloft, they have to exert a force equal to their weight. So 5000 kg of birds must exert 5000 kg of down force. I think where it gets a little weird is that the air is spreading out and moving around. But the net force still has to be equal.

I believe I saw a video on TV or YouTube showing an RC helicopter flying in a hover above a scale. You could see the weight of the helicopter was indeed showing up on the scale via downforce.

 

I agree, if there is no weight on the deck, there is no weight to lift.

 

I see it differently. If the airplane is carrying the air (moving it forward, Undisturbed, through the external air mass) it is carrying the weight of the internal air and any steady state mass within it. Does a fish tank weigh less if a frog swims off of the bottom? However, once the bird flies out of the window and into the slipstream the airplane becomes lighter. Also, if the bird jumps up, for a moment the weight is removed, but this is different, because the airplane actually gets heavier when the bird pushes against the floor. And again heavier for an instant when he lands. It all balances out. There is no free ride.

About the treadmill....

 

Everything inside the fuselage has mass, even the air. If the birds flew in the plane, the mass of the whole is still the same, much like the frog in the fishbowl swimming up off the bottom, they are supported by the molecular substance of the air. If you fart inside an closed airplane, you get lighter, but the airplane doesn't.

 

To answer the OPs question- yes. The plane weighs 5,000 kg less in theory and ignoring airflow issues assiciated with bird wing lift/drag/down draught.

 

Not true. Gas has Mass. The bird is supported by the air, the air is supported by the plane.

 

I concur.

 

Come on guys, this is pretty simple and Lou has it right. There is some force that supports the birds and that force is spread out over the floor of the area as an increase in air pressure. There is no free lunch...

 

Why just "over the floor"? In a sealed and pressurized cabin, wouldn't the force be spread more or less equally over all surfaces?

 

Lou and other like minds have it wrong.

 

air ( not in motion ) cannot support an object that is heavier that air... reason balloons fly and everything else is stuck to the ground

 

This is the correct answer.

The key is that the birds are actively supporting their weight by flying. This exerts a force equal to their weight on the air which exerts that same force on the inner hull of the aircraft. No different than if the birds were supporting themselves with little sticks holding them up off of the floor. Their weight is transmitted through a fluid, in this case air, to the object supporting the fluid. A similar question would be, if you have a swimming pool sitting on a scale, then floated a boat in the pool, would the weight shown on the scale change? Of course it would.

Now, If you took dead birds and hung them from the ceiling of the cabin, and then dropped them to the floor, the plane would "weigh" 5000kg lighter until they came back to rest on the floor.

 

that's why birds flap their wings

 

No, since the birds need to generate their own weight in downward force to lift off...

 

For the birds to fly they have to impart momentum to the air in the cabin. That movement of air results in a differential pressure in the cabin. That is, the pressure on the floor is higher than the pressure on the other surfaces. The air in the cabin is in motion if the birds are flying.

If you go look up wing theory you will find that the wing of an aircraft imparts downward momentum on the air around it and that creates lift. Same thing with the wings of birds in the cabin. Downward motion of air results in a pressure differential that is spread over the large area of the cabin.

In short, the weight of the birds are supported by a very small pressure differential between the floor and roof of the cabin.

Sorry if you just don't get it Cheesy... Lou had it right the first time..

 

Someone call adam and jamie.

 

You are correct. This is simple physics at an elementary level. The fish tank analogy was a good one, here's another:
Imagine a large windowless container sitting on a scale, which reads 10,000 lbs. What you don't know is that there are 5,000 lbs of birds sitting on the floor of the container. Now imagine the birds all take off inside the container and hover at a constant height, does the scale suddenly read only 5000 lbs?. Of course not. (Can the scale fluctuate as the birds initiate flight, and decent? Yes, but that is another topic as the op is referencing steady-state conditions).

 

really, very entertaining view of flight. makes for good fiction

show me the math on how you get to your answer... I would offer to show how I get to my answer, but since my answer is being discredited, someone with credentials will need to show you to make the answer credible for you and others :=)

 

over simplified...YES...

 

This is how Mr. Spock was able to get the Klingon Battle Cruiser off the ground with the two whales in it (floating in water) in that episode that took place in San Francisco.

Things do not weigh anything when they are floating in something.

For example, if you floated a steel ball bearing in a vessel of molten lead, the vessel of molten lead would not weigh more than without the ball bearing.

My adopted cat is looking intently at a goldfish in a fishbowl right now.